Visual display matrix

ABSTRACT

Apparatus for controlling a rectangular matrix of gas-filled discharge tubes comprises two further rectangular matrices of pulse transformers. One further matrix controls the row conductors of the discharge tube matrix whilst the other further matrix controls the column conductors of the discharge tube matrix. The two further matrices are controlled in such a manner that selected discharge tubes may be switched on or off as required.

United States Patent [72] Inventor George Richard Hoffman Sale, England [21 Appl No. 789,476

[22] Filed Jan. 7, 1969 [45] Patented June 1, 1971 [73] Assignee Ferranti Limited Hollinwood, England [32] Priority Jan. 13, 1968 [33] Great Britain [54] VISUAL DISPLAY MATRIX [50] Field of Search 340/166 [5 6] References Cited UNITED STATES PATENTS 2,833,920 5/1958 Buley 340/166X Primary Examiner-Harold l. Pitts Attorney-Cameron, Kerkam and Sutton ABSTRACT: Apparatus for controlling a rectangular matrix of gas-filled discharge tubes comprises two further rectangular matrices of pulse transfonners. One further matrix controls 6Claims8mawing Figs the row conductors of the discharge tube matrix whilst the [52] US. Cl 340/166, other further matrix controls the column conductors of the 340/ I 67, 340/176 discharge tube matrix. The two further matrices are controlled [51] lnt.Cl H04q 1/00, in such a manner that selected discharge tubes may be H04q 3/00 switched on or off as required.

Pu/se Genemf/ng T [DU/$8 I 0/56/2049? l (mm/t l Morn/x PATENIEUJUN Hen 3,582,891

sum 5 [1F 6 PATENIEDJUN Han 31582691 SHEET 6 [1F 6 VISUAL DISPLAY MATRIX This invention relates to visual display devices, and is particularly concerned with the control of a large array of gasfilled cold-cathode discharge tubes each of which may be struck or extinguished to produce a visual display of the required form.

The type of visual display device under consideration is one in which a plurality of cold-cathode gas-filled discharge tubes are arranged in the rows and columns of a rectangular matrix with one electrode of each tube in a row connected to an associated row conductor and one electrode of each tube in a column connected to an associated column conductor such that a selected tube may be struck or extinguished by variation of the potentials applied to the appropriate row conductor and column conductor. A series resistance is connected between one electrode of each tube and the appropriate row or column conductor. A visual display device of this type is hereafter referred to as being of the type specified.

It is well known that, with a gas-filled discharge tube, once a striking voltage has been applied to the tube, causing a discharge to form, the discharge will be maintained even if the voltage across the tube is reduced to a running voltage. The discharge may be extinguished by reducing the voltage across the tube to a still lower value, the extinguishing voltage, and the discharge will remain extinguished even when the voltage across the tube returns to the running" value.

An object of the invention is to provide apparatus for controlling a visual display device of the type specified.

According to the invention apparatus for controlling a visual display device of the type specified comprises a. a first pulse generating circuit having a number of outputs each of which is connected to a separate row conductor of the discharge tube matrix and operableto apply to each conductor a fixed voltage of one polarity of which may be superimposed a pulse of either said one or the op posite polarity; I

b. a second pulse generating circuit having a number of outputs each of which is connected to a separate column conductor of the discharge tube matrix and operable to apply to each conductor a fixed voltage of said opposite polarity on which may be superimposed a pulse of either the one or the opposite polarity;

each pulse generating circuit comprising a plurality of pulse transformers connected in a further matrix and operable to provide the fixed voltage and the required pulses, each transformer having its primary winding connected between a row wire and a column wire of said further matrix and the potentials of each row wire and each column wire and said further matrix being controlled by a separate switching device; and

c. means for selectively actuating each switching device in such a manner that the required combination of pulses is applied to the selected one of the discharge tubes to cause it to strike or become extinguished as the case may be.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of the display device and control means;

FIG. 2 is a circuit diagram of part of a display device of the type specified;

FIG. 3 is a circuit diagram of part of a pulse generating circuit according to the invention;

FIG. 4a and 4b illustrate the pulse waveforms obtained on actuation of the pulse generating circuits;

FIGS. 5a and 5b illustrate the waveforms used to control the display device; and

FIG. 6 is a modified form of FIG. 2.

FIG. 1 shows how the display device 10 is arranged as a rectangular matrix having its row conductors connected to a first pulse generating circuit 11 and having its column conducts connected to a second pulse generating circuit 12. The two pulse generating circuits are themselves controlled by the actuating means 13.

Referring now to FIG. 2, a plurality of cold-cathode gasfilled discharge tubes G are connected into a matrix of column conductors 1: to x and row conductors y to y,,. Each discharge tube is connected in series with a resistor R between one row conductor and one column conductor as shown. One end of each row and column conductor is connected to one of the two pulse circuit as will be described below.

FIG. 3 shows part of the pulse generating circuit for driving I the column conductors of the matrix of FIG. 2. The pulse generating circuit for driving the row conductors of the matrix is very similar. I

A constant current source, depicted schematically at C, supplies current to the emitter of a'pnp transistor Z and also to the emitters of 32 pnp transistors, column transistors TX, to TX The base of transistor Z is connected to earth potential and its collector to a potential V, which is more negative than earth. Each of the transistors TX, to TX has its collector connectedto a separate column wire X, to X and the other end of each column wire is connected through a resistor RX, to RX to a voltage %V,.' The base of each column transistor TX to TX is connected to the collector of a driver transistor TL, to TI The collector of each column transistor is also connected through a diode DX to DX poled as shown, to a voltage V,.

Thirty-two NPN row transistors TY to TY, each have their collector electrode connected to a separate row wire Y, to Y and the other end of each row wire is connected through a resistor RY, to RY to earth. The emitter of each row transistor is connected to the voltage %V,, and the base of each row transistor is connected to an input terminal [Y to IY The collector of each row transistor is also connected through a diode DY, to DY to earth.

Connected to one column conductor and one row conductor at each of the 1,024 intersections is the primary winding of a pulse transformer T in series with a diode DT poled as shown. The secondary winding of each pulse transformer has one end connected to a voltage -%V,,,. The other end of each secondary winding is connected to a different one of the column conductors x,, to x, of the discharge tube matrix.

The other pulse generating circuit, which drives the row conductors of the, discharge tube matrix is very similar to that shown in FIG. 3 and described above. The only differences are that the polarities of all the potentials and the diodes are reversed, and each transistor is replaced by a transistor of the opposite conductivity kind.

The values of V, and V, are defined as follows: V is the sustaining voltage which, when connected across the discharge tube-resistor combination, will maintain the discharge tube in either the struck or extinguished condition. V the trigger voltage, is the voltage by which V, has to be increased to cause a discharge tube to strike, or the voltage by which V, has to be decreased to cause a discharge to be extinguished.

' The operation of the pulse circuit of FIG. 3' will now be described with reference to FIGS. 3 and.4.

When all the column transistors TX, to TX are nonconducting, a current I flows from the constant current source C through transistor Z which acts as a current sink. Since no current is flowing in either the column wires or the row wires, the diodes DT in series with the primary windings of the pulse transformers are reverse biased by a voltage kV,. The catching diodesDX are also reverse biased by the same voltage.

If an input signal is applied to, say,,terminal IY sufficient to cause transistor TY to saturate, then the collector of TY, will be held at the voltage %V,. This causes a small current to flow along the row wire Y and removes the reverse bias from the diodes DT to UT If now an input signal is applied to terminal 'IX', such that driver transistor IX saturates, then a small current-will flowalong the column wire X,, and removes the reverse bias from the diodes DT', DT etc., connected to the X, column conductor. Apart form the removal of the reverse bias across these diodes, the circuit containing the pulse transformer T, behaves as illustrated by FIG. 40. FIG. 4a i) shows the switching pulse applied to the driver transistor IX,, and FIG. 40 ii) shows the current flowing through the primary winding of transformer T and shows how it builds up approximately linearly until it reaches a maximum value close to the current available from source C. The voltage across the transformer primary, shown in FIG. 4a iii), rises to a maximum value and remains at that value until the current in the winding ceases to rise, when the voltage drops to zero.

When the driver transistor IX, is switched off, a reverse potential of 'V, volts appears across the primary winding of the transformer T,. This voltage is defined by the collector voltage of transistor TY, (i.e. /zV, volts) and the catching voltage on diode DX, (i.e. V, volts). The current flowing in the transformer primary falls approximately linearly to zero, when the voltage pulse ceases. Transistor TY may then be switched off. The diode DY is provided to protect the transistor TY,. The output waveform X is identical with the waveform of FIG. 40 iii).

The above description considered the case where the transformer current built up to a maximum value, and remained at that value until transistor IX was switched off. The waveforms of FIG. 4b illustrate what happens if IX, is switched off as soon as the current reaches a maximum value, as shown in FIG. 4b 1'). The primary voltage waveform at FIG. 4b iii) shows that when the initial positive-going voltage pulse ceases it is carried on immediately into the negative-going pulse. The output waveform X is identical to the waveform of FIG. 4b iii). The corresponding secondary voltage waveform Y for the other pulse circuit is also shown at FIG. 4b iv). This is the inverse of the X waveform.

In operation, one pulse generating circuit, say the circuit driving the column wires of the discharge-tube matrix, is used in both the modes described above, so that either X or X waveforms may be produced as desired. The other pulse generating circuit is only required to produce one type of waveform, namely that shown at FIG. 4b iv). The duration of the input signals to the terminals IX determine the waveform produced.

Consider now the case of a gas-filled discharge tube which requires a voltage of 350 volts to be applied across it and its series resistor in order to strike the discharge. The normal running voltage (V,,,) is 250 volts, and the tube may be extinguished by reducing this to 150 volts. The trigger voltage (V,) is thus 100 volts. FIG. a shows how such a tube may be caused to strike by the application of the X waveform to the appropriate column conductor and the Y waveform to the corresponding row conductor. Initially the conductors are at voltages of l25 volts and +125 volts respectively. The voltage of 250 volts across the discharge tube-resistor combination is not sufficient to cause the tube to strike. When the initial voltage pulse appears, at time t,, the voltage across the tube falls to 150 volts. At the end of this pulse, at time 1 the voltage rises immediately to 350 volts, and the tube strikes between time t, and time When the voltage pulse ends at time the voltage returns to 250 volts, and the discharge is maintained.

If the tube has been initially struck, then it would have been extinguished between times t and times t but restruck between times t, and

FIG. 5b shows how a discharge tube which is struck may be extinguished. In this case the X waveform is replaced by the X waveform as shown. At time t, the voltage across the discharge tube-resistor combination is reduced to 150 volts, and the discharge is extinguished between times t, and 1,. At time t, the voltage rises to 300 volts, but this is not sufficient to restrike the discharge. At time although the voltages on the column and row conductors change, the overall voltage remains at 300 volts. Finally at time the voltage falls to 250 volts.

As stated above, the choice of X or X waveforms is determined by the duration of the input signals to the pulse circuit. Since the most likely application of a gas-discharge tube matrix having US. Pat. No. 1,048,576 cross points (i.e. 1,024 x 1,024) will be for a computer readout system, these input signals will be determined by the computer itself.

One of the problems which occurs with circuits of the type described above is that of ringing due to the inductances and the stray capacitances. This may be considerably reduced by means of the circuit modification described below.

FIG. 6 shows a modified form of part of the circuit of FIG. 3. The modification involves the connection of an extra transistor TC to each of the column wires. The transistor shown is a pnp type having its emitter connected to the potential %V, and its collector connected through a catching diode DC, poled as shown, to the column wire X. The base of the transistor is connected to an input terminal IC.

By using the additional transistor as a clamp, the timing control of the pulse generating circuit is shifted from the input terminals IX to the clamp input terminals IC. Each column transistor TX in the matrix is now switched in exactly the same way as those of the other pulse generating circuit that is, the transistor is switched on for a predetermined time, and then switched off.

If the clamping transistor is switched off from time t, to time t;, the X output waveform of FIG. 5a is produced, resulting in a selected discharge tube being switched on, as described above. The clamping transistor is switched on again at time i to prevent ringing.

If a discharge tube is to be switched off, then the X output waveform is required. To produce this, the clamping transistor TC appropriate to the required column wire of the pulse circuit is switched on at some time during the initial positivegoing part of the output waveform, that is between time t and time t,. The action of the clamping transistor is to prevent the wavefonn from going negative by an amount in excess of %V,, and hence the output waveform will be held at %V,,,. The current in the transformer primary winding is maintained by current from the transistor TC. When, at some later time transistor TC is switched off, the transformer primary voltage is able to fall to -V,, where it is held by the diode DX. Hence the negative-going part of the X waveform of FIG. 5b is produced. The clamping transistor is switched on again at time t, to prevent ringing, as before.

In order to reduce the physical size of the transformers it is possible to apply a direct-current bias to each transformer. An alternative, and preferable, arrangement is to use transformer cores having an air gap. This linearizes the inductance and also increases the number of ampere-turns which may be used before saturation effects predominate.

Since, with the modified embodiment of FIG. 6, the current through the column transistors TX is never allowed to reach a limiting value, the constantcurrent source C is no longer es sential. If it is removed, along with the sink transistor Z, then the emitter of each column transistor TX may be connected to earth or some other suitable potential. In this case the column transistors TX will have a lower output impedance than before.

An alternative switching arrangement uses only the X waveform described above, in place of the two different waveforms previously required. The required effect is produced applying the same waveform to both the row and column conductors defining the selected discharge tube, but with one waveform delayed in time with respect to the other. If for example, the X waveform is applied to one conductor, say the row conductor, commencing at a time t,, and the same waveform is applied to the column conductor commencing at time 1 then the selected discharge tube will strike (t, and t, refer to the time intervals of the X waveform, as illustrated in FIG. 5a). If, however, the waveforms are reversed so that the X waveform is applied to the column conductor at time t, and to the row conductor at time t,, then the discharge tube will be extinguished. The control arrangement necessary is simpler than that already described, since only one waveform need be produced, but a simple time delay element is required. However, this alternative switching arrangement is necessarily slower than the original arrangement because of the time delay introduced.

The pulse transformers preferably have bifilar winding, since this prevents the production of spurious pulses by transformers located other than at the selected cross-point.

The reference above to the numbers of rows of columns in a pulse generating circuit or discharge tube matrix are only by way of examples, as are the various potentials mentioned.

What I claim is:

1. Apparatus for controlling a visual display device of the type specified, which includes a. a first pulse generating circuit having a number of outputs each of which is connected to a separate row conductor of the discharge tube matrix and operable to apply to each conductor a fixed voltage of one polarity on which may be superimposed a pulse of either said one or the opposite polarity,

b. a second pulse generating circuit having a number of out puts each of which is connected to a separate column conductor of the discharge tube matrix and operable to apply to each conductor a fixed voltage of said opposite polarity on which may be superimposed a pulse of either the one or the opposite polarity,

each pulse generating circuit comprising a plurality of pulse transformers connected in a further matrix and operable to provide the fixed voltage and the required pulses, each transformer having its primary winding connected between a row wire and a column wire of said further matrix and the potentials of each row wire and each column wire of said further matrix being controlled by a separate switching device, and

c. actuating means for selectively actuating each switching device in such a manner that the required combination of voltage pulses is applied to the selected one of the discharge tubes to cause it to strike or to become extinguished as the case may be.

2. Apparatus as claimed in claim 1 in which one of said pulse generating circuits is operable to produce either of two alternative pulse waveforms and the other of said pulse generating circuits is operable to produce only one of said two waveforms.

3. Apparatus as claimed in claim 2, in which the shape of the pulse waveform produced by said one pulse generating circuit is determined by the time for which current flows along the required row and column conductors of said further matrix to the selected pulse transformer.

4. Apparatus as claimed in claim 3, in which the duration of the current flow is determined by the switching device connected to the required conductors.

5. Apparatus as claimed in claim 3, in which the duration of the current flow along at least one of the two conductors is determined by a clamping device actuated by the actuating means and operable to limit the voltage swing of said conduc- 6. Apparatus as claimed in claim 5 in which said clamping device is a transistor.

3 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, Dated June 1,

Inventor(s) George Richard Hoffman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Cover page, item [31] "2029" should read --2029/68. Column 4, line 1, "'U.S. Pat. No. should be deleted.

Signed and sealed this 5th day of October 1971.

(SEAL) Attest:

EDWARD M.FLET CHER,JR. ROBERT GOTTSCHALK Attestlng Officer Acting Commissioner of Patents 

1. Apparatus for controlling a visual display device of the type specified, which includes a. a first pulse generating circuit having a number of outputs each of which is connected to a separate row conductor of the discharge tube matrix and operable to apply to each conductor a fixed voltage of one polarity on which may be superimposed a pulse of either said one or the opposite polarity, b. a second pulse generating circuit having a number of outputs each of which is connected to a separate column conductor of the discharge tube matrix and operable to apply to each conductor a fixed voltage of said opposite polarity on which may be superimposed a pulse of either the one or the opposite polarity, each pulse generating circuit comprisIng a plurality of pulse transformers connected in a further matrix and operable to provide the fixed voltage and the required pulses, each transformer having its primary winding connected between a row wire and a column wire of said further matrix and the potentials of each row wire and each column wire of said further matrix being controlled by a separate switching device, and c. actuating means for selectively actuating each switching device in such a manner that the required combination of voltage pulses is applied to the selected one of the discharge tubes to cause it to strike or to become extinguished as the case may be.
 2. Apparatus as claimed in claim 1 in which one of said pulse generating circuits is operable to produce either of two alternative pulse waveforms and the other of said pulse generating circuits is operable to produce only one of said two waveforms.
 3. Apparatus as claimed in claim 2, in which the shape of the pulse waveform produced by said one pulse generating circuit is determined by the time for which current flows along the required row and column conductors of said further matrix to the selected pulse transformer.
 4. Apparatus as claimed in claim 3, in which the duration of the current flow is determined by the switching device connected to the required conductors.
 5. Apparatus as claimed in claim 3, in which the duration of the current flow along at least one of the two conductors is determined by a clamping device actuated by the actuating means and operable to limit the voltage swing of said conductor.
 6. Apparatus as claimed in claim 5 in which said clamping device is a transistor. 